Leave a Message sequence diagram

1. Caller keys in extension (shown as single call to dial)
2. Connection calls system's find_mailbox
3. The Mailbox object on the right is returned
4. Connection invokes get_greeting
   (greeting is passive; doesn't appear on diagram)
5. Connection plays appropriate greeting
6. Telephone reads msg from user, passes it to connection
7. Telephone reads hangup, passes it to connection
8. Connection constructs Message, adds it to mailbox